Added documentation of stage/TU allocation

http://codereview.appspot.com/6199058/



git-svn-id: http://skia.googlecode.com/svn/trunk@3888 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrDrawState.h b/src/gpu/GrDrawState.h
index 386aebd..357f864 100644
--- a/src/gpu/GrDrawState.h
+++ b/src/gpu/GrDrawState.h
@@ -36,6 +36,11 @@
      * the last enabled stage. The presence or absence of texture coordinates
      * for each stage in the vertex layout indicates whether a stage is enabled
      * or not.
+     *
+     * Stages 0 through GrPaint::kTotalStages-1 are reserved for setting up
+     * the draw (i.e., textures and filter masks). Stages GrPaint::kTotalStages 
+     * through kNumStages-1 are earmarked for use by GrTextContext and 
+     * GrPathRenderer-derived classes.
      */
     enum {
         kNumStages = 4,